1. Technical Field
The present invention relates to a position measuring instrument for determining an absolute position.
2. Background Information
Absolute position measuring instruments in which absolute position information is derived from a code track having code elements disposed in succession in a measuring direction are being used increasingly in many fields. The code elements are provided in a pseudo-random distribution, so that a defined number of successive code elements each form one bit pattern. Upon displacement of the scanning unit relative to the code track by a single code element, a new bit pattern is already formed, and over the entire measurement range that is to be detected in absolute form, a series of different bit patterns is available.
One such sequential code is known as a chain code or pseudo-random code (PRC).
A position measuring instrument with a pseudo-random code is explained in German patent disclosure DE 10 2006 010 161 A1, for example. A plurality of code tracks extending parallel to one another are provided, each of which has the same series of code elements. By scanning one of these code tracks, the absolute position in one measuring direction is determined, and by scanning a further code track, the absolute position in a second direction perpendicular to the first direction is determined.
An absolute position measuring instrument that is a point of departure for the present invention is described in European patent disclosure EP 1 468 254 B1. The position measuring instrument has a code which includes a series of code elements in succession in the measuring direction, and each code element in turn has two subregions, which are complementary to one another and are disposed in succession in the measuring direction. This code is scanned by a scanning unit that has a plurality of detectors. In an evaluation unit, a comparison output signal is formed from the scanning signals from the subregions of one code element, and a check is made as to whether the comparison output signal is above or below a reference value, and as a function of that, a bit “0” or “1” is derived as code information for the corresponding code element.